The revision of the Energy Performance of Buildings Directive (EPBD), together with the Renovation Wave Strategy defined by the European Commission to promote long-term renovation strategies for the existing housing stock, will lead to the compulsive application of dynamic simulation software to small commercial or residential buildings in substitution of the oversimplified seasonal methodology. Thus, this work aimed to evaluate the effects of different improvements in the envelope of a non-insulated existing building on its dynamic thermal behaviour by using the EnergyPlus™ program. The analysed different envelopes included non-insulated slab floor in contact with the soil, suspended non-insulated and insulated slab floors, the introduction of exterior wall and ceiling insulation and substitution of simple glazing with double glazing windows and appropriate fixed and operable shade devices. The simulation results for typical winter and summer sunny days indicate that the high thermal inertia of the floor in contact with the soil resulted in a better thermal performance when compared with the suspended floor, while the addition of adequate external thermal insulations, together with a relatively high south facing glazing area can lead to a nZEB, provided that the appropriate window awnings and exterior operable blinds are correctly implemented and used.